1. Field of the Invention
The present invention relates to shelters, particularly shelters for individuals or groups of people caught in the open in inclement weather especially when there is a danger of lightning.
2. Summary of the Prior Art
Persons caught in the open during inclement weather may be at risk of being struck by lightning. The present applicant proposed in GB 2 332 458A a lightning shelter having a cover supported by an electrically-conductive frame and a floor of electrically-conductive mesh. The frame comprised a plurality of hollow metal poles which are anchored to the ground by earthing or grounding spikes. As shown in FIG. 1, the mesh floor 10 was welded to a collar 12 surrounding the lower end of one frame pole 14 which was anchored to a grounding spike 16. The collar 12 was fixed to the metal pole by bolts 18.
The present applicant has appreciated that extremely high voltage gradients may be generated when grounding a lightning bolt through a spike in the ground. In fact, the spike dissipates to ground a current pulse associated with the lightning strike, generating a high voltage potential centred on the spike. Points on the ground spaced from the spike will be at a much lower potential, giving rise to a voltage gradient, with voltage decreasing rapidly in a radial direction from the spike. The voltage gradient may be lethal to humans and animals since an electric shock. across vital organs e.g. heart may be produced when one leg is on the ground at a much higher potential than the other (so-called step voltage).
In accordance with the present invention, there is provided a shelter comprising: an elongate electrically-conductive member for supporting a canopy, the elongate electrically-conductive member being configured as a lightning conductor; an electrically-conducting floor; and a coupling member for electrically connecting the elongate electrically-conductive member to the electrically-conducting floor, wherein the coupling member includes a flange extending laterally away from the elongate electrically-conducting member, over the electrically-conductive flooring, to provide an enlarged footprint of engagement with the electrically-conducting floor.
The footprint should be of such a size as to ensure complete electrical conduction between the elongate member and the floor. The flange may surround the elongate electrically-conductive member, and may be substantially annular. By avoiding sharp points or corners, voltage concentrations capable of starting electrical breakdown (sparks) are prevented. In this way, the flange may ensure a uniform potential (equi-potential) is produced across the electrically-conducting floor, thereby preventing the occurrence of a lethal xe2x80x9cstep voltagexe2x80x9d during a lightning strike.
The electrically-conductive flooring may comprise a metal mesh. The coupling member may comprise a further flange which opposes and is movable relative to the aforementioned flange. The flanges may thus define a pair of jaws which are configured to engage opposite sides of the electrically-conductive flooring. The flanges may be urged together using bolts to provide a clamping action. Electrical resistance between the flange(s) and the electrically-conductive flooring may be below 100 mxcexa9; in this way explosive resistive heating caused by a high current lightning pulse may be avoided.
For larger shelters, the electrically-conductive flooring may comprise a first and a second metal mesh, with one overlapping the other at the coupling member. In this way, the flange may be used with an opposing part to clamp the first and second metal meshes together, urging one into intimate contact with the other.
Each metal mesh may be supplied from a roll, and may therefore have a length which is greater than its width. In the case of first and second metal meshes from a roll, one metal mesh may be aligned with its roll direction at an angle to that of the other. In other words, the meshes may be positioned so that their longitudinal axes are at an angle to each other. The angle may be 90xc2x0. In this way, a plurality of metal meshes may be used and coupled together to cover an area which could be greater than 500 m2 e.g. large enough to cover a tennis court.
A portion of the elongate electrically-conductive member which extends away from the coupling member may have an electrically-insulating cover, e.g. polyurethane sheath. The cover may be configured to provide protection for an individual from a touch voltage of up to 9 kV e.g. by employing a cover of thickness of about 3 mm. The touch voltage is the voltage difference between a position 2 m from ground level up the elongate electrically-conducting member to ground potential. When lightning strikes a conductor a touch voltage of up to 9 kV may be generated, and thus an individual may need to be insulated from it.
The electrically-conducting floor may further comprise a layer of insulation on top of the metal mesh. The layer of insulation may be at least 25 mm thick to prevent spikes on sports shoes (e.g. golf shoes) from penetrating the layer of insulation and making contact with the metal mesh. The layer of insulation may comprise rubber, e.g. granulated rubber bonded with resin.
The layer of insulation may be configured to provide for rigidity, for example by comprising a hard material such as concrete.
The shelter may comprise a foundation member under the coupling member which is configured to provide ballast for the elongate electrically conductive member. For example, the foundation member might comprise a dense material such as concrete. The foundation member may help maintain the integrity of the shelter by reducing movement of the portion of the electrically conductive member which is below ground, thereby preventing the generation and enlargement of holes or voids at the bottom end of the electrically conductive member. The foundation member may surround and extend laterally away from the elongate electrically conductive member at least as far as the bolts that clamp the flanges together.
The elongate electrically-conductive member may comprise a pole and a spike member driveable into the ground, the spike member being configured to anchor one end of the pole to the ground when in use. One end of the pole may be a push fit onto an end of the spike member which remains exposed when driven into the ground. One part of the clamping member may be coupled (e.g. welded) to the spike member.
The elongate electrically-conductive member may further comprise a elongate connecting member configured at one end to be a push fit onto the lower end of the pole and at the opposite end to be a push fit onto the upper end of the spike member. The connecting member may help the pole and spike member to register with each other during assembly. In addition, the connecting member may provide a robust connection between the spike member and the pole.
The shelter may comprise at least one further electrically-conducting member configured to be driveable into and anchor in the ground around the shelter""s perimeter. When anchored in the ground, the at least one further electrically-conducting member may be used to measure the electrical potential of the ground to thereby test the performance of the shelter.